1. Field
The present invention relates generally to audio signal processing and, more particularly, to digital to analog converters.
2. Description of Related Art
Design of digital audio systems involves a number of trade-offs that result in the system having certain characteristics that are beneficial and certain characteristics that are less desirable. For example, the range of the system can be sacrificed to allow for simpler circuit designs. Also, device size can be minimized by sacrificing other parameters depending on the desired characteristics of the system. One component of a digital audio system that affects its performance is the digital to analog converter (DAC). In particular, the DAC component may adversely affect the linearity of the digital audio system response because of nonlinearities within the DAC component.
A DAC typically includes a stage that converts a current to an analog voltage using an operational amplifier and a feedback resistor. If the feedback resistor includes nonlinear characteristics, then the voltage output from the DAC will also have a nonlinear response to the digital audio signal being input. There are a variety of semiconductor manufacturing techniques capable of producing the feedback resistor in different configurations. One such technique is a silicide block poly resistors which are highly linear over a wide range of inputs. However, manufacturing these types of resistors require additional processing steps that complicates their construction and increases cost. An alternative type of resistor is an n-well resistor that is relatively easier to manufacture but that has some nonlinear properties. For example, the resistance of the n-well resistor varies according to the voltage signal across it.
The nonlinearities of the digital audio system and its components may vary over time, temperature, or other variables. Additionally, nonlinearities may be introduced during the manufacturing process. There remains a need therefore, for an approach to address the linearity performance of a digital audio system that includes nonlinear resistors in the current-to-voltage converter of an audio DAC.